I assume my soil needs some type of fertilizer but how do I know which fertilizer to choose and how much to apply. FM

Fertilizer can be an intimidating topic. Because of this, many people tend to go with a “balanced” fertilizer; something with equal amount of the big three nutrients; nitrogen, phosphorus, and potassium. The problem with this strategy is that not knowing the nutrient content of your soil leaves you with two probable outcomes; you will apply either too much or too little. Both of these options fail to give your landscape what it needs.

To better understand soil nutrients let’s look to an illustration involving a lawn mower. To run properly, lawn mowers need gas and oil unless you have an electric mower, in which case it only needs electricity, but let’s get back to gas powered lawn mowers. When the mower is operating properly, gas gets consumed to power the mower while the oil keeps the mower lubricated and operating properly. No gas… no go. Too much gas…fire hazard. Too much or too little oil…engine doesn’t work right. Both gas and oil must be present in the proper amounts for the engine to run the way it should. Fortunately, on the lawn mower we can open the gas cap and do a visual check to see if more gasoline is needed and utilize a dipstick to check oil levels to determine if we need to add more oil.

In this illustration, gasoline is analogous to nitrogen and oil is analogous to phosphorus and potassium. For plants, nitrogen is the fuel they need to grow. Phosphors and potassium are like the oil, these nutrients must be present at the proper level to enable the chemistry to work like it should.

To monitor soil nutrient levels, the soil test is our dipstick. It allows us to find out whether we need to add nutrients or not. Without a soil test we are just guessing. Just imagine how well your lawnmower would work if you just guessed at fuel and oil levels. It probably wouldn’t work so well. Same thing with fertilizer. When we guess at the nutrient levels, we likely guess wrong. We have information on our website in the Hot Topics section about how to go about getting your soil tested. Now back to fertilizers.

When shopping for fertilizer, the first thing you will notice is that there are a lot of options and a lot of numbers. The numbers tell you the nutrient content of the fertilizer. For example, if the fertilizer is labeled 10-20-10, the first number is the percentage of nitrogen, the second is the percentage of phosphorus, and the last number is the percentage of potassium. So, this fertilizer would contain 10% nitrogen, 20% phosphorus, and 10% potassium. The rest of the ingredients would be inert or neutral binding agents.

Now, let’s assume you collected your soil sample, took it to the OSU Extension, and you now have your test results. Oftentimes soils tests will show that your soil is low on nitrogen while phosphorus and potassium levels were appropriate or possibly high. In this case, you would only need to add nitrogen to your soil. Furthermore, this means that if you were purchasing fertilizer that contained phosphorus and potassium, you would not only be paying for something you didn’t need, but by continuing to boost your phosphorus and potassium levels you could actually be worsening growing conditions in your soil.

Oftentimes the recommendation from a soil test is to add 1 lb. of nitrogen per 1000 square feet. A fertilizer such as Urea which contains 49% nitrogen would be an appropriate selection in this case. Now you need to do a little math.

Urea is marked 49-0-0 which means it is 49% nitrogen and 0% phosphorus or potassium. With a nitrogen level of 49%, this means one pound of this fertilizer would be almost half nitrogen. To get 1 pound of nitrogen, you would need two pounds of Urea.

The next questions we usually get is where to set the drop level on the fertilizer to drop 2 pounds of Urea per 1000 square feet. The best way to figure this out is to do a little experiment.

Start by stepping off a 1000 square foot area in your lawn. This would be approximately a 33’ by 33’ square. If your lawn is 100‘ wide, just use a 10’ by 100’ portion or some variation of that.

Now, put enough fertilizer in your spreader so that you have one pound of nitrogen. For the 49-0-0 in our example, this would be 2 pounds. If you were using 10-20-10, that would be 10 pounds of fertilizer. Next, take a guess at where to set your spreader to deliver this amount over your test area. Start on the low side as you begin spreading the fertilizer on the test area. You may need to adjust as you go or even do a second test to confirm you are dropping fertilizer at the intended rate. The good news is that if you use the same fertilizer next time, you won’t have to repeat this process, just duplicate your settings on the spreader.

If this still sounds a little confusing, we have several instructional videos on our website (www.tulsamastergardeners.org). Just go to the Lawn and Garden Help Section and click on Soil. I have every confidence you can figure it out. See you in the garden!

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org.

Photo Courtesy: Bill Sevier